A conventional filter insertion type of an optical branch apparatus comprises an input port for receiving a signal light having a wavelength of, for instance, 1.3 .mu.m and a monitor light having a wavelength of, for instance, 1.55 .mu.m, a plurality of output ports for supplying terminals with branched signal lights, optical waveguides connected in turn by branch ports formed at a plurality of stages to provide an input optical waveguide to be connected to the input port and a plurality of output optical waveguides to be connected to the plurality of output ports, and an optical filter provided to intersect the input optical waveguide at a predetermined angle between the input port and the branch port of the first stage, so that the monitor light is reflected in the direction of the input port by the optical filter, while the signal light is transmitted through the optical filter to be transmitted through the branched optical waveguides to the plurality of output ports.
A conventional optical fiber line-managing system comprises an optical fiber line monitor for supplying the signal light and the monitor light through the optical fiber line to the filter insertion type of the optical branch apparatus. The optical fiber line monitor comprises an OTDR (Optical Time Domain Reflectmeter) for emitting the monitor light to be supplied through the optical fiber line to the optical branch apparatus and receiving the monitor light which is reflected from the optical filter of the optical branch apparatus to detect an abnormal state of the optical fiber line, an optical signal-emitting unit for emitting the signal light to be supplied through the optical fiber line to the optical branch apparatus, and an optical coupler for coupling the monitor and signal lights to be transmitted through the optical fiber line to the optical branch apparatus and receiving the reflected monitor light to be received in the OTDR.
In operation, the monitor light is emitted from the OTDR through the optical coupler and the optical fiber line to be supplied to the optical branch apparatus. In the optical branch apparatus, the monitor light is reflected through the optical fiber line and the optical coupler by the optical filter, and the reflected monitor light is received in the OTDR. Consequently, an abnormal state is detected in the OTDR in accordance with the receipt of the reflected monitor light.
In the conventional filter insertion type of an optical branch apparatus, however, there are disadvantages in that the reflected signal light is increased to decrease the reflection attenuation at 48 dB to 53 dB, and then a reflected light which has a variety of phases and amplitudes is increased to be returned to the optical signal-emitting unit, because the optical filter is provided to intersect the input optical waveguide.
Further, the conventional optical branch apparatus has a disadvantage in that it is easy to occur noise, because a lasing mode of a laser light is to be instable, when the reflected light is incident to a resonator of a semiconductor laser which is a light source.
Even further, the conventional optical branch apparatus has disadvantages in that if a process accuracy is low, the optical loss is increased, and a substrate is broken and/or cracked in a washing process of the optical branch apparatus, because the slit into which the optical filter is inserted is formed by a groove process.
Where the reflect attenuation (Q) is calculated based on a ratio of an optical power (P,) of the signal light supplied to an incident terminal of an optical device from the light source such as a semiconductor laser, etc. and an optical power (P.sub.2) of the reflected light from the incident terminal, as follows: EQU Q(dB)=-10 log.sub.10 (P.sub.2 /P.sub.1) (1)
As shown by the above expression (1), the reflected light is large when the reflection attenuation is small. For reliability, the reflection attenuation needs generally over 50 dB.